The late (21-90 days) lesions caused in the heart and blood vessels of rats consuming allylamine were studied by light and electron microscopy. These lesions consisted of extensive left ventricular scarring, with the formation of left ventricular aneurysms, and endocardial thickening similar in many respects to that seen in endocardial fibroelastosis, and with focal areas of cartilaginous metaplasia. Vascular lesions were characterized by marked fibromuscular intimal proliferation.